FEB 25, 202657 MINS READ
The vinylidene fluoride hexafluoropropylene tetrafluoroethylene terpolymer is a three-component fluoroelastomer whose performance is governed by precise monomer ratios and molecular architecture. The terpolymer typically comprises 30–70 mol% tetrafluoroethylene (TFE) units, which provide the polymer backbone with outstanding chemical resistance and thermal stability 13. Hexafluoropropylene (HFP) content generally ranges from 1.5–40 mol%, contributing chain flexibility and lowering the glass transition temperature to maintain elastomeric behavior at sub-ambient conditions 10. Vinylidene fluoride (VDF) units, present at 8.5–35 mol%, introduce reactive sites essential for peroxide or bisphenol-based crosslinking, enabling the formation of three-dimensional networks during vulcanization 711.
The fluorine content of commercial VDF-HFP-TFE terpolymers is typically maintained between 64–69 wt%, a critical parameter that directly influences fuel oil resistance and swelling behavior in aggressive solvents 918. Higher fluorine content correlates with reduced hydrocarbon solubility and enhanced resistance to amine-containing fluids, a key requirement for automotive fuel system seals exposed to modern biofuel blends 14. The Mooney viscosity (ML 1+10 at 100°C) of these terpolymers ranges from 10 to 200, with values between 40–80 preferred for injection molding and extrusion processes to balance processability and green strength 72.
Molecular weight distribution and chain branching are controlled during aqueous emulsion polymerization by adjusting initiator concentration, chain transfer agents, and polymerization temperature (typically 60–120°C) 12. The presence of perfluoro(alkyl vinyl ether) comonomers such as perfluoro(methyl vinyl ether) or perfluoro(propyl vinyl ether) in some formulations (0.1–11 mol%) further enhances low-temperature flexibility and reduces compression set at elevated service temperatures 120.
Key structural features include:
Crosslinking is essential to transform the thermoplastic terpolymer into a thermoset elastomer with requisite mechanical properties and solvent resistance. Three primary vulcanization systems are employed: peroxide cure, bisphenol cure, and polyol cure, each offering distinct advantages depending on application requirements 48.
Peroxide-curable VDF-HFP-TFE terpolymers are formulated with organic peroxides such as 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane or dicumyl peroxide at 0.5–6 parts per hundred rubber (phr) 918. The peroxide decomposes at 160–180°C, generating free radicals that abstract hydrogen atoms from VDF units and initiate crosslinking via carbon-carbon bond formation 4. Co-agents such as triallyl isocyanurate (TAIC) or triallyl cyanurate (TAC) are added at 2–8 phr to increase crosslink density and improve tensile strength (from 8 MPa to 15–18 MPa) and elongation at break (150–250%) 411.
Peroxide cure systems offer excellent heat resistance (continuous service up to 200°C) and superior compression set resistance (<25% after 70 hours at 200°C per ASTM D395 Method B) compared to bisphenol systems 9. However, peroxide-cured elastomers exhibit slightly lower chemical resistance to strong bases (e.g., 50% NaOH at 100°C) due to residual hydrocarbon structures from co-agents 4.
Bisphenol AF (4,4'-(hexafluoroisopropylidene)diphenol) is the preferred curative for VDF-rich terpolymers, used at 2–6 phr in combination with quaternary phosphonium salts (e.g., benzyltriphenylphosphonium chloride) at 0.5–2 phr as accelerators 117. The crosslinking mechanism involves nucleophilic substitution of vinylidene fluoride units by phenoxide anions, forming ether linkages and eliminating HF 2. Cure temperatures range from 160–180°C with press cure times of 10–20 minutes, followed by post-cure at 200–230°C for 4–24 hours to complete crosslinking and remove volatiles 11.
Bisphenol-cured elastomers demonstrate outstanding resistance to polar solvents (methanol, ethanol, acetone) and fuel oils (ASTM Fuel C, biodiesel B20), with volume swell typically <15% after 168 hours at 23°C 14. Compression set values are moderate (30–40% after 70 hours at 175°C), making this system suitable for static seals and gaskets 9.
Polyol cure systems employ multifunctional alcohols or amines (e.g., hexamethylenediamine carbamate) at 1–4 phr, activated by metal oxides (MgO, CaO) and quaternary ammonium salts 14. These systems provide rapid cure kinetics (scorch time >5 minutes at 120°C, t90 <15 minutes at 170°C per ASTM D2084) and excellent low-temperature flexibility (Tg as low as -25°C) 16. However, polyamine-cured elastomers exhibit reduced thermal stability above 180°C due to amine oxidation 8.
Critical formulation parameters include:
Crosslinked VDF-HFP-TFE terpolymers exhibit a comprehensive property profile that balances mechanical strength, elasticity, and environmental resistance. Tensile strength typically ranges from 10 to 20 MPa (ASTM D412, Die C, 500 mm/min), with ultimate elongation between 150% and 300% depending on filler loading and crosslink density 19. Shore A hardness is adjustable from 60 to 90 by varying carbon black content (10–50 phr) and plasticizer addition (0–10 phr of processing oils or low-molecular-weight fluoropolymers) 1118.
The elastic modulus at 100% elongation (M100) ranges from 3 to 8 MPa, providing sufficient stiffness for sealing applications while maintaining conformability to mating surfaces 9. Tear strength (ASTM D624, Die C) is typically 20–45 kN/m for carbon black-reinforced compounds, with needle-shaped or fibrous fillers (average fiber diameter <5 μm, length 40–80 μm) increasing tear resistance by 30–50% through crack deflection mechanisms 1.
VDF-HFP-TFE elastomers demonstrate excellent thermal stability, with continuous service temperatures up to 200°C and intermittent exposure capability to 230°C 49. Thermogravimetric analysis (TGA) reveals onset decomposition temperatures (5% weight loss) between 380°C and 420°C in nitrogen atmosphere, with primary degradation occurring via chain scission and HF elimination above 450°C 7. The glass transition temperature (Tg) ranges from -20°C to -35°C depending on HFP content, enabling low-temperature sealing performance down to -40°C for automotive applications 1416.
Compression set resistance is a critical parameter for dynamic seals, with peroxide-cured formulations achieving <25% set after 70 hours at 200°C (ASTM D395 Method B, 25% compression) 9. Post-cure protocols (200–230°C for 4–24 hours) are essential to minimize compression set by completing crosslinking reactions and removing residual curatives and volatiles 11.
The high fluorine content (64–69 wt%) of VDF-HFP-TFE terpolymers confers outstanding resistance to a broad spectrum of chemicals, including:
However, VDF-HFP-TFE elastomers exhibit limited resistance to strong bases (e.g., 50% NaOH at 100°C causes >20% swell and surface cracking after 168 hours) and polar aprotic solvents (e.g., dimethylformamide, dimethyl sulfoxide) 48. Amine resistance is moderate, with tertiary amines causing 10–25% volume swell depending on amine basicity and temperature 4.
Low-temperature performance is governed by the Tg and crystallinity of the terpolymer. Formulations with 20–40 mol% HFP exhibit Tg values of -25°C to -35°C, enabling TR-10 (temperature at 10% retraction, ASTM D1329) values of -15°C to -30°C 16. The addition of plasticizers such as perfluoropolyether oils (5–15 phr) can further depress Tg by 5–10°C, though at the cost of reduced tensile strength and increased extractables 11.
Dynamic mechanical analysis (DMA) reveals a storage modulus (E') of 5–15 MPa at 23°C (1 Hz), decreasing to 1–3 MPa at 150°C, indicating retention of elastomeric behavior across the service temperature range 20. Tan δ peaks at Tg are typically 0.3–0.6, reflecting moderate damping characteristics suitable for vibration isolation and noise reduction in automotive applications 16.
VDF-HFP-TFE terpolymers are synthesized via aqueous emulsion polymerization, a process that enables precise control over monomer composition, molecular weight, and particle size distribution 12. The polymerization is conducted in a stirred autoclave reactor at 60–120°C and 1–3 MPa pressure, using water as the continuous phase and fluorinated surfactants (e.g., ammonium perfluorooctanoate at 0.05–0.5 wt% based on water) as emulsifiers 122.
Redox initiator systems are preferred for their ability to generate free radicals at moderate temperatures, minimizing chain transfer and branching. Common initiators include ammonium persulfate (APS) combined with sodium metabisulfite or ferrous sulfate at molar ratios of 1:0.5 to 1:2 12. The polymerization rate is controlled by initiator concentration (0.01–0.5 wt% based on monomer), with higher concentrations yielding lower molecular weight polymers (Mn = 50,000–150,000 g/mol) and faster conversion rates (80–95% in 4–12 hours) 27.
Chain transfer agents such as diethyl malonate, isopropanol, or carbon tetrachloride (0.05–1 wt% based on monomer) are employed to regulate molecular weight and introduce reactive end groups for subsequent crosslinking 812. The monomer feed ratio is adjusted continuously during polymerization to compensate for reactivity differences (rTFE ≈ 3–6, rHFP ≈ 0.2–0.5, rVDF ≈ 1–2), ensuring compositional uniformity throughout the polymer chain 12.
Post-polymerization, the latex is coagulated by addition of aluminum sulfate, calcium chloride, or dilute sulfuric acid, causing polymer particles to agglomerate and separate from the aqueous phase 212. The coagulated polymer is washed repeatedly with deionized water (3–5 cycles) to remove residual surfactants, salts, and unreacted monomers, achieving extractables <0.5 wt% (ASTM D297, acetone extraction) 7. Drying is performed in vacuum ovens at 80–120°C for 12–24 hours, reducing moisture content to <0.1 wt% to prevent hydrolysis during storage and compounding 2.
Critical process parameters include:
Compounding VDF-HFP-TFE terpolymers requires careful selection of fillers, curatives, and processing aids to achieve target properties while maintaining processability. A typical peroxide-cure formulation comprises:
| Org | Application Scenarios | Product/Project | Technical Outcomes |
|---|---|---|---|
| NOK CORPORATION | Bi-directional rotation sealing systems in automotive powertrains and industrial machinery requiring continuous fluid sealing during both normal and reverse rotational operations. | Bi-directional Oil Seal | Incorporates needle-shaped or fibrous fillers (average fiber diameter ≤5 μm, length 40-80 μm) in VDF-HFP-TFE five-component polymer, achieving 30-50% increase in tear resistance through crack deflection mechanisms while maintaining tensile strength of 10-20 MPa and elongation of 150-300%. |
| ASAHI GLASS COMPANY LIMITED | High-temperature automotive sealing components, fuel system gaskets, and chemical processing equipment requiring rapid manufacturing cycles and superior heat resistance up to 200°C continuous service. | Crosslinkable Fluoroelastomer with Hydroxyphenyl Groups | Enhanced crosslinking reactivity with cure times reduced from 20 minutes to 8-12 minutes at 170°C, achieving compression set <25% after 70 hours at 200°C and tensile strength of 15-18 MPa through hydroxyphenyl-containing ethylenically unsaturated compounds (0.1-5 mol%) combined with quaternary phosphonium salts. |
| 3M INNOVATIVE PROPERTIES COMPANY | Automotive fuel system components including fuel hoses, tank seals, and vapor management systems exposed to modern biofuel blends (E85, B20) and oxygenated fuels at temperatures up to 150°C. | Fluoroelastomer for Fuel Management Systems | Optimized terpolymer composition with 30-70 mol% TFE, 1.5-40 mol% HFP, and 8.5-35 mol% VDF achieving fluorine content of 64-69 wt%, delivering volume swell <15% in biodiesel B20 and excellent gas permeation resistance with improved bonding to silicone substrates. |
| NOK CORPORATION | Fuel oil system sealing materials for automotive and industrial applications, including gaskets, O-rings, and static seals contacting petroleum-based fuels and lubricants at elevated temperatures. | Peroxide-Curable Fuel Oil Sealing Material | Peroxide-crosslinked VDF-HFP-TFE terpolymer (67-69 wt% fluorine) with carbon black (5-20 m²/g specific surface area) and bituminous fillers, achieving excellent fuel oil resistance, compression set <30% at 175°C, and metal corrosion resistance without metal oxide acid acceptors. |
| DAIKIN INDUSTRIES LTD | Chemical processing equipment housings, filter components, and piping systems exposed to high-temperature (>180°C) and high-pressure aggressive chemical solutions requiring long-term dimensional stability and durability. | High-Rigidity Fluoropolymer for Chemical Processing | TFE-HFP-perfluoro(propyl vinyl ether) terpolymer with controlled melt flow rate providing enhanced wear resistance, solvent crack resistance, high-temperature rigidity retention above 180°C, low hydrogen permeability, and superior tensile creep resistance under repeated loading. |